In the early days of Greek drama there was only one actor on stage, playing all the different parts. The introduction of the chorus, then, was largely practical. It provided the audience with a distraction while the sole actor went off stage to change or prepare for his next role.

In due course, however, the role of the chorus became more closely related to the structure and tone of the play. In providing a commentary on the action, for instance, the chorus would serve to establish a much closer connection between the audience and what was happening on stage. The chorus could also help prepare the audience for dramatic shifts. This would allow the playwright more effectively to control the overall mood of the play.

The position of the chorus in Greek drama was by no means static, and it developed gradually over time. In Prometheus Bound, for instance, Aeschylus allows the chorus to become part of the action on stage rather than simply commenting upon it. His theatrical innovation drew the ire of Aristotle, among others, who accused him of diminishing the chorus's stature.

Yet we must never forget that Greek theater was communal in nature. In allowing the chorus to become an intrinsic part of the action, Aeschylus was also enabling the audience to experience a deeper moral engagement with the play. In effect, the chorus was no longer confined to the stage; it also interacted with the audience.

Classical drama, according to Aristotle, started as exclusively choral, in performances known as "dithyrambs". The addition of individual actors was a later development. Choruses are used differently in tragedy and comedy. They sometimes speak as normative voices, reminding the audience of shared standards being violated by the characters. At other times, they may add narrative elements.

In the actual staging of ancient tragedy, choruses, like those of modern opera, dance, adding impressive visual components to a drama.

In some plays choral odes are clearly connected to plot development, but in others they serve as poetic and visual interludes with only tenuous connections to the main plot.

The role of the chorus in drama is much like the role of the narrator of a novel or story.  The chorus provides some structure to the narrative and can provide information and commentary that need not be in the actors' lines.  The chorus can also provide foreshadowing or humor.  The chorus acts as a kind of liaison between the audience and the actors, giving the audience a lens through which to view the action of the drama, allowing the audience to identify with a group that is "outside" the action, the way a frame story operates, with a narrator telling a story within a story, giving the reader a narrative one step removed.  This is an ancient technique in drama, for example, in Oedipus Rex.  However, there are modern instances of the chorus.  One example that comes to mind is Little Shop of Horrors, which uses a singing "chorus" to great effect, with narration and humorous commentary.